For proper vision, eyes need to focus exactly on images. The two main determinants of refraction are focus power of cornea and the eye's axis length. In normal eyes, images are formed exactly on retina (emmetropia), but farsightedness (hyperopia) is the situation in which, the eye forms images in the backside of retina and nearsightedness (myopia) is a situation in which images are formed in front of retina, and it may have two reasons: 1) the lens convexity is more powerful than normal and 2) the eye axis is longer than normal (axial myopia).